There has been known a switching power supply unit generating a required output voltage by repeatedly switching on and off an input voltage impressed on a coil to step up the input voltage, and by rectifying and smoothing the resultant voltage with a diode and a capacitor. This switching power supply unit is controlled by a feedback system in which a feed back voltage proportional to the output voltage balances out a reference voltage. Thus, the output voltage is controlled to remain at a predetermined level (constant-voltage control).
Such constant-voltage control of a switching power supply unit is carried out by an IC designed for this purpose (said IC hereinafter referred to as IC). However, the IC tends to raise the output voltage above the predetermined level in cases where, for example, a voltage division circuit generating the feedback voltage or an external rectification diode have (has) becomes open on account of connection failure, and where the output circuit of the switching power supply unit has become short-circuited, because the feedback circuit judges then that the output voltage is insufficient. As a consequence, the switching power supply unit presents a problem that it outputs abnormally high voltage and/or excessively large current.
In order to protect the switching power supply unit, some switching power supply units utilizing a step-up transformer are adapted to monitor the output voltage to detect an abnormal load condition (e.g. arc discharge). This type of switching power supply unit detects an abnormal load condition when the currently monitored value is below a predetermined threshold value indicative of abnormality of a load and is larger than the last monitored value. When such abnormal load condition is detected twice in succession, the duty of the PWM signal controlling the power supply unit is minimized (Japanese Patent Application Laid Open No. 2001-008446).
In the switching power supply unit as disclosed in the cited literature, it is necessary to detect the level of the output voltage that is currently below the predetermined threshold level but is larger than the previously monitored level. Thus, monitoring of the output level is necessary at least three times for detection of abnormality, which can result in a delay in the detection.
Moreover, it is necessary to define an abnormal level of the output voltage to be detected (i.e. threshold of the output voltage) in accordance with an individual load condition, which, however, requires tedious procedures.